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Interstellar Flight by Particle Beam Revisited

Beamed propulsion is the classic solution to the mass ratio problem in interstellar flight. Rather than pushing more and more fuel to get your payload to another star system, you leave the fuel behind. Robert Forward’s vast lightsail proposals come immediately to mind, but in 2001 physicist Geoffrey Landis proposed propulsion by particle beam, with energy delivered from the Solar System to the departing spacecraft.

The notion is this: a charged particle beam is accelerated and focused, then neutralized in charge to prevent the beam from expanding as it travels due to electrostatic repulsion. When the particles reach their target, they are re-ionized and reflected by a magnetic sail, which Landis originally conceived as ‘a large superconducing loop with a diameter of many tens of kilometers.’

The particle-beam idea seems to solve two problems with lightsails: First, a light beam provides a relatively inefficient energy source, demanding huge power facilities and thus driving up the cost. Some of Forward’s proposals were mind-boggling, calling for laser power up to an astounding 7.2 terawatts. The latter was for Forward’s most audacious concept, a round-trip manned mission to Epsilon Eridani.

The second issue is that laser-pushed sails call for extremely large lenses to deliver a tightly collimated beam to the receding starship. The lens for Forward’s Epsilon Eridani mission would be fully a thousand kilometers in diameter, dwarfing our manufacturing capabilities into the forseeable future. Add to these problems the difficulty of manufacturing ultra-thin films in space, not to mention deploying them as sails, and the particle beam emerges as an increasingly attractive alternative.

Landis has now revised his original particle beam ideas to incorporate new technologies. Specifically, he proposes using techniques developed by Robert Winglee at the University of Washington to reduce the physical structure of the sail. The concept is to create a magnetic sail through the use of mini-magnetosphere plasma propulsion (M2P2). From the paper:

By shrinking the physical structure of a magnetic sail, the invention of the mini-magnetosphere plasma propulsion, or ‘M2P2’, has brought the idea of a particle-beam-pushed sail closer to reality. The particle-beam is reflected by a magnetic field. In the mini-magnetosphere, the magnetic field is inflated to a large area by injection of a plasma into the magnetic field, and hence large magnetic field areas are possible with only a small physical structure.

M2P2 in actionWinglee’s ideas, tuned for missions within the Solar System, assume an
on-board supply of propellant to replenish the plasma, which gradually leaks into space. Landis’ idea is to replace the plasma from the particle beam itself. ‘…Thus the mini-magnetosphere will self-inflate with the particle beam, resulting in no requirement for on-board propellant except, possibly, for initial inflation of the magnetic field.’

Image: An artist’s impression of a mini-magnetosphere deployed around a spacecraft. Plasma or ionized gas is trapped on the magnetic field lines generated onboard, and this plasma inflates the magnetic field much like hot air inflates a balloon. The mini-magnetosphere is then blown by the plasma wind from the Sun or, in Geoffrey Landis’ interstellar concept, by a particle beam sent from the Solar System. Credit: Robert Winglee, University of Washington.

Two more particle beam advantages: The magnetic sail provides braking by creating drag against the solar wind from the destination star, and also becomes a shield against interstellar dust.

For more on mini-magnetosphere plasma propulsion, see the M2P2 page at the University of Washington. The revised Landis paper is “Interstellar Flight by Particle Beam,” in Acta Astronautica Vol. 55, pp. 931-934 (2004). For useful background on magnetic sails and braking, see D. G. Andrews and R. M. Zubrin, “Magnetic Sails and Interstellar Travel,” International Astronautical Federation Paper IAF-88-5533, Bangalore, India, October 1988. A useful cost study of interstellar mission options was performed by Dana Andrews in “Cost Considerations for Interstellar Missions,” Paper IAA-93-706 (1993).

Comments on this entry are closed.

  • James M. Essig November 16, 2006, 1:19

    The idea of using a particle beam to accellerate a manned spacecraft to relativistic velocities is fascinating. If the crew members were positioned inside sealed water filled capsules, it may be possible to achieve accellerations of 10s of Gs if not much greater accelleration because the capsules would function simmilar to the flight suits used by fighter pilots but allow even greater accellerations then flight suits currently permit. If a craft can be designed to accellarate at 100 Gs for a few weeks earth time, the terminal velocity of the craft (after the end of this few week period) would be close to the speed of light. Obviously, a benefit of rapid accelleration is such that the beam energy need not be as precisely focused because the beam does not need to reach a distant target of otherwise smaller angular size.

    However laser beams may still be useful for accellerating manned interstellar spacecraft provided that during the initial accelleration phase, they power a photovoltaic cell powered ion or electron rocket. Once relativistic velocities are achieved, the laser beam could be used for the force it exerts directly on a light sail. For example, using a Newtonian mechanics approximation, if a beam is exerting a force of 100,000 Newtons on a craft that is travelling 10 million meters per second, the result is an effective mechanical power of 1 trillion watts or about 1.3 billion horse power which is about one order of magnitude greater than the maximum power generated by the space shuttle. If the resulting rate of kinetic energy gain of 1 trllion joules/ sec for the ship continues for 3 years or for about 100 million seconds, the kinetic energy gain by the ship would be the equivalent of converting about 1 metric ton of matter into pure energy. At this rate, after 10 years of constant accelleration, a 100 metric ship would be travelling at a velocity greater then 25 percent of C. If advances in very high strength materials such as carbon nanotube materials comes to fruition, a light sail that is an effectively porous grid might be made which is much lighter than a monolithic or continuos sheet . If the holes in the sheet are on the order of a few times smaller than the wavelength of the impinging light, a sheet which is almost entirely empty space may make a good visible light or IR reflector thus resulting in mass savings.

    Although I make obvious points here to which I do not claim to be the originator, perhaps the notion of using a variety of propulsion techniques within one manned interstellar spacecraft for relatively near term manned local interstellar missions should be addressed periodically using technological concepts that allready exist but which may simply need to be scaled up to meet the propulsion power requirements.

    Once we learn how to greatly scale up beam power, be it a photonic beam or a massive particle beam, and if and when we learn how to extend the human life span on Earth, then the really fun part of meaningful manned flight throughout the milky way galaxy and perhaps beyond may begin.

    However, we must take the baby steps of first of learning how to reach the nearest stars. I feel that a judiciuos selection of multiple propulsion methods using further refined existing technologies in a single manned craft might be the way to go.

  • Adam November 16, 2006, 5:17

    Hi James

    Jordin Kare’s laser micro-sail beam propulsion might be the best option for our first probes and ships as it has relatively low power requirements (GW not TW) and is a logical extension of beamed power technology which could first be deployed powering space elevators.

  • James M. Essig November 17, 2006, 0:21

    Hi Adam;

    Thank you for your comments regarding my response of Nov. 16. at 1:19. I think that the micro-sail beam propulsion concept might optionally be powered by inflatable space-based solar energy concentrators that focus concentrated sunlight onto photovoltaic cells to power the laser portions of the assemblage. I have read about photovoltaic cells under development which can operate under several hundred fold concentrated sunlight at Earth’s distance from the sun. Using inflatable solar energy concentrators along with correspondingly smaller solar cells would seem to allow the mass specific power output of an electrically powered laser’s power source to be greatly reduced compared to such a system using solar panels that collect sunlight at ambient concentration.

    The inflatable reflectors would be vunerable to puncture by micro-meteriods and space debris. However, the air density required to keep the reflectors pressurized to adequately tension the device’s reflective membrane inorder for the membrane to act as a sunlight concentrator is greatly reduced in a vacuum compared to standard atmospheric pressure on Earth. The absolute pressure required to keep such structures properly inflated decreases significantly with increases in the inflatable’s size.

    I would think that concentrated sunlight from a spacebased reflector having an area of a few square kilometers operating on photovoltaic cells that have an effeciency of about 34 percent (which are currently under development) would be able to provide approximately 1 GW of electrical power input into the laser beam generator. Even though electrically powered lasers are not 100 percent effecient, scaling up the reflector surface area by a factor of say a few should allow for the generation of a 1 GW laser beam to drive the microsails.

    Note that there has been significant research into space-based inflatable solar energy concentrators, not only by NASA, but also through the Aerospace firm
    L’Garde (atleast I think that’s how its spelled).

    I would not be suprised if the U.S., other industrialized nations, or the international community as a whole were able to design and field a realistic inflatable reflector-based power source simmilar to the one I described above within the next 2 to 3 decades for the purpose of driving the microsails. Note, also, that a spacebased high mass-specific power output inflatable solar energy concentator could be used to power the particle beam generator as discussed in Paul Gilster’s article above.

  • Adam November 17, 2006, 5:24

    Hi James

    L’Garde’s knowledge base on inflatables is encouraging. Their materials are space-tested and hardy enough for long-duration use in space. Geoffrey Landis has published a study on concentrator SPS using high-efficiency cells (35%) with very low mass overheads. He has published a design which generates 3 GW (in space) with a system mass of just 1,300 tons. The array itself masses just 800 tons, while 500 tons goes to support structure. The array is composed of ~ 30,000 individual collector/rectenna units which beam power independently of each other, thus eliminating heavy power distribution cabling, sliprings, brushes etc. which are unavoidable in monolithic SPS designs.

    Such an in-space power supply would be superlative in applications to beamed power for deep-space missions and the first interstellar probes. With suitable microwave technology and a global power-grid such designs could also power the planet. Massive incentives exist to build power-arrays in space once power-to-mass ratios drop sufficiently.

  • James M. Essig November 21, 2006, 0:38

    Hi Adam;

    I am curious about the concept of using beamed energy to accellerate a manned spacecraft to mildly relativistic velocities within the range of say .05C to about .15C at which point a nuclear fusion rocket system would take over propulsion once the craft left the useful range for aiming accuracy of the beam. I would think that an efficient fusion rocket propulsion system could then bring the craft up to approximately .3C to .35C with a fusion fuel to payload mass ratio of somewhere between one and ten times that of the space shuttle before launch.

    If a solid material sail where used to propel the spacecraft during the beamed power propulsion leg of the journey and the sail was composed mostly if not almost entirely of materials capable of undergoing exothermic nuclear fusion reactions, perhaps the sail material could be processed enroute and also used for fuel to power the spacecraft. If the sail were made from carbon fibers or carbon nanotubes, a fusion sequence using carbon nuclei as reactants could possibly work. However, I am not sure of the temperature range and/or pressures required to cause carbon nuclei to fuse at any sufficient rate in any workable carbon fusion reactor used for ship propulsion. Perhaps if someone or some organization invents a strong durable fiber sail somehow made from molecularly stable compressed hydrogen, the fusion temperatures could be significantly lower, perhaps reducing the technical hurdles that must be overcome in developing a practical, working fusion rocket.

    Another, much talked about, alternative method of interstellar travel that I find interesting is to use beamed energy to accellerate the various proposed types of interstellar ramjets to the mldly relativistic velocities required for the jets to reach the critical values at which point the ramjet can then start to collect and burn interstellar hydrogen and/or helium from the interstellar medium to produce significant and useful levels of thrust.

    In terms of known candidates for rocket fuels, obviously the best fuels would involve matter and antimatter for mutual annihilation, and because of the relativistic rocket equation: delta V = C{tanh[(Isp/C)ln(Mo/M1)]}, efficient antimatter rockets may not need an initial boost from a beam, although I can imagine some scenarios where such might be benificial such in as the design of craft able to reach highly relativistic velocities.

    Now if we can only get NASA, The Department of Energy, The Department of Defense, and/or the international community to invest more in the development and comercialization of useful amounts of antimatter in spacebased production facilities (somewhat removed from low Earth orbit for safety reasons), I think many of us space enthusiasts would be thrilled including myself. Perhaps the assembly of efficient antimatter producing particle accellerators powered by the photovoltaic systems you mentioned as being described in the study published be Geoffrey Landis would be one of the best uses of efficient and low cost access to Earth orbit.

    Again Adam, thanks for the information contained in the study that Geoffrey Landis has published. I continue to look forward to further commentary from you and correspondance with you.